A wide range of medical procedures involve placing objects, such as sensors, tubes, catheters, dispensing devices, and implants, within the body. Real-time imaging methods are often used to assist doctors in visualizing the object and its surroundings during these procedures. In most situations, however, real-time three-dimensional imaging is not possible or desirable. Instead, systems for obtaining real-time spatial coordinates of the internal object are often utilized.
Many such position sensing systems have been developed or envisioned in the prior art. Some systems involve attaching sensors to the internal object in the form of transducers or antennas, which can sense magnetic, electric, or ultrasonic fields generated outside of the body. For example, U.S. Pat. Nos. 5,697,377 and 5,983,126 to Wittkampf, whose disclosures are incorporated herein by reference, describe a system in which three substantially orthogonal alternating signals are applied through the subject. A catheter is equipped with at least one measuring electrode, and a voltage is sensed between the catheter tip and a reference electrode. The voltage signal has components corresponding to the three orthogonal applied current signals, from which calculations are made for determination of the three-dimensional location of the catheter tip within the body.
Similar methods for sensing voltage differentials between electrodes are disclosed by U.S. Pat. No. 5,899,860 to Pfeiffer; U.S. Pat. No. 6,095,150 to Panescu; U.S. Pat. No. 6,456,864 to Swanson; and U.S. Pat. Nos. 6,050,267 and 5,944,022 to Nardella, all of whose disclosures are incorporated herein by reference.